The present invention relates to an orientation layer for liquid crystal molecules and a process for preparation thereof. The invention also relates to an optical compensatory sheet comprising a transparent substrate, an orientation layer and an optically anisotropic layer comprising discotic liquid crystal molecules in this order. The invention further relates to a liquid crystal display of an STN mode. The invention furthermore relates to a process of alignment of discotic liquid crystal molecules at an average inclined angle in the range of 50xc2x0 to 90xc2x0.
A liquid crystal display of an STN mode comprises a liquid crystal cell of an STN (Super Twisted Nematic) mode, two polarizing elements arranged on each side of the liquid crystal cell, and one or two optical compensatory sheets (phase retarders) arranged between the liquid crystal cell and the polarizing element.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecule. Each of the substrates has an orientation layer, which has a function of aligning the rod-like liquid crystal molecule. The rod-like liquid crystal molecules are twisted by using a chiral agent. A twist angle of the molecules is in the range of 90xc2x0 to 360xc2x0.
Without use of an optical compensatory sheet, a liquid crystal display of an STN mode displays an image colored blue or yellow because of birefringence of rod-like liquid crystal molecules. The blue or yellow color is inconvenient for not only a black and white image but also a color image. An optical compensatory sheet has a function of removing color from an image to display a bright and clear image. The optical compensatory sheet also has a function of enlarging a viewing angle of a liquid crystal cell. A stretched birefringent film has usually been used as the optical compensatory sheet. Japanese Patent Provisional Publication Nos. 7(1995)-104284 and 7(1995)-13021 disclose an optical compensatory sheet made of a stretched birefringent film for a liquid crystal display of an STN mode.
An optical compensatory sheet comprising an optically anisotropic layer on a transparent substrate has been proposed to be used in place of the stretched birefringent film. The optically anisotropic layer is formed by aligning discotic liquid crystal molecules and fixing the aligned molecules. The discotic liquid crystal molecules usually have large birefringence. The discotic liquid crystal molecules also have various alignment forms. Accordingly, an optical compensatory sheet obtained by using the discotic liquid crystal molecule has a specific optical characteristic that cannot be obtained by the conventional stretched birefringent film. The optical compensatory sheet using the discotic liquid crystal molecule is disclosed in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1. However, the disclosed optical compensatory sheet is designed to be used in a liquid crystal display of a TN (Twisted Nematic) mode.
The optical compensatory sheet containing the discotic liquid crystal molecules should be redesigned to be used in a liquid crystal display of an STN mode. In a liquid crystal cell of the STN mode, rod-like liquid crystal molecules are aligned according to a super twisted birefringent mode at a twist angle of larger than 90xc2x0. The liquid crystal display of the STN mode can display a clear image of a large volume according to a time-sharing addressing method even though the display has a simple matrix electrode structure (having no active matrix such as a thin film transistor or a diode).
The discotic liquid crystal molecules should be essentially vertically aligned (homogeneously aligned) to optically compensate the liquid crystal cell of the STN mode. The discotic liquid crystal molecules are preferably further twisted.
Japanese Patent Provisional Publication No. 9(1997)-26572 discloses an optical compensatory sheet in which discotic liquid crystal molecules are twisted. The drawings of Japanese Patent Provisional Publication No. 9(1997)-26572 further illustrate that discotic liquid crystal molecules are essentially vertically aligned. According to the method disclosed in Japanese Patent Provisional Publication No. 9(1997)-26572, discotic liquid crystal molecules are essentially vertically aligned by coating the molecules on a glass substrate, drying them, and applying magnetic field to them along a direction parallel to the substrate.
The method disclosed in Japanese Patent Provisional Publication No. 9(1997)-26572 requires a specific apparatus for magnetic alignment of the discotic liquid crystal molecules. Further, it is technically difficult to reproduce the magnetic alignment.
On the other hand, rod-like liquid crystal molecules used in a liquid crystal cell have been investigated to align the molecule essentially vertically (homeotropic alignment). For example, a liquid crystal display of a vertical alignment (VA) mode uses an orientation layer having a function of essentially vertically aligning rod-like liquid crystal molecules. In the liquid crystal display of the VA mode, rod-like liquid crystal molecules are essentially vertically aligned while not applying voltage to the cell, and are essentially horizontally aligned while applying voltage to the cell. Various orientation layers have been proposed to align rod-like liquid crystal molecules.
However, it is technically difficult to align discotic liquid crystal molecules uniformly (monodomain alignment) from an interface facing an orientation layer to another interface facing the air by using an orientation layer proposed to align rod-like liquid crystal molecules. Most of the orientation layers having a function of aligning rod-like liquid crystal molecules are not effective in aligning discotic liquid crystal molecules.
An object of the present invention is to provide an orientation layer having a function of aligning liquid crystal molecule (particularly discotic liquid crystal molecules) vertically.
Another object of the invention is to provide an optical compensatory sheet suitable for a liquid crystal display of an STN mode.
A further object of the invention is to provide a liquid crystal display of an STN mode that can display a clear image of a high contrast, in which blue or yellow color caused by birefringence of rod-like liquid crystal molecule is reduced.
A furthermore object of the invention is to provide a method for vertical, uniform and stable alignment of discotic liquid crystal molecules.
The present invention provides an orientation layer for aligning liquid crystal molecules provided on a substrate, wherein the orientation layer contains an acrylic copolymer or a methacrylic copolymer comprising repeating units represented by the formula (I) and repeating units represented by the formula (II) or (III): 
in which R1 is hydrogen or methyl; R2 is hydrogen, a halogen atom or an alkyl group having 1 to 6 carbon atoms; M is proton, an alkali metal ion or an ammonium ion; L0 is a divalent linking group selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94SO2xe2x80x94, an alkylene group, an alkenylene group, an arylene group and a combination thereof; R0 is a hydrocarbon group having 10 to 100 carbon atoms or a fluorine atom substituted hydrocarbon group having 1 to 100 carbon atoms; Cy is an alicyclic group, an aromatic group or a heterocyclic group; m is 10 to 99 mole %; and n is 1 to 90 mole %.
The invention also provides a process for the preparation of an orientation layer which comprises the steps of: coating a solution of an acrylic copolymer or a methacrylic copolymer comprising repeating units represented by the formula (I) and repeating units represented by the formula (II) or (III) on a substrate to form a coated layer; drying the coated layer; rubbing a surface of the coated layer; and heating the coated layer at 50 to 300xc2x0 C. in this order.
The invention further provides an optical compensatory sheet comprising a transparent substrate, an orientation layer and an optically anisotropic layer in this order, said optically anisotropic layer comprising discotic liquid crystal molecules, wherein the orientation layer contains an acrylic copolymer or a methacrylic copolymer comprising repeating units represented by the formula (I) and repeating units represented by the formula (II) or (III), said discotic liquid crystal molecules being aligned at an average inclined angle in the range of 50xc2x0 to 90xc2x0.
The invention furthermore provides a liquid crystal display comprising a liquid crystal cell of an STN mode, two polarizing elements arranged on each side of the liquid crystal cell and one or two optical compensatory sheets arranged between the liquid crystal cell and the polarizing elements, wherein the optical compensatory sheet comprises a transparent substrate, an orientation layer and an optically anisotropic layer in order, said transparent substrate being adjacent to the polarizing element, said optically anisotropic layer comprising discotic liquid crystal molecules, said orientation layer containing an acrylic copolymer or a methacrylic copolymer comprising repeating units represented by the formula (I) and repeating units represented by the formula (II) or (III), said discotic liquid crystal molecules being aligned at an average inclined angle in the range of 50xc2x0 to 90xc2x0, and said discotic liquid crystal molecules being twisted at a twist angle in the range of 90xc2x0 to 360xc2x0.
The invention still furthermore provides a method of alignment of discotic liquid crystal molecules, which comprises forming an optically anisotropic layer comprising discotic liquid crystal molecules on an orientation layer containing an acrylic copolymer or a methacrylic copolymer comprising repeating units represented by the formula (I) and repeating units represented by the formula (II) or (III) to align the discotic liquid crystal molecules at an average inclined angle in the range of 50xc2x0 to 90xc2x0.
In the present specification, the term xe2x80x9caverage inclined anglexe2x80x9d means an average of angles between discotic planes of discotic liquid crystal molecules (or long axes of rod-like liquid crystal molecules) and a surface of a transparent substrate (or a surface of an orientation layer). The present specification refers to alignment of liquid crystal molecules at an average inclined angle in the range of 50xc2x0 to 90xc2x0 as essentially vertical alignment of the molecules.
The present inventors have succeeded in obtaining essentially vertical, uniform and stable alignment of liquid crystal molecules by using an orientation layer containing a specific (meth)acrylic copolymer. The copolymer has repeating units having a side chain containing a hydrocarbon group, a side chain containing a fluorine atom substituted hydrocarbon group or a cyclic structure directly attached to a main chain. The orientation layer is particularly effective in aligning discotic liquid crystal molecule.
An optical compensatory sheet suitable for a liquid crystal display of an STN mode is now obtained by using the discotic liquid crystal molecules of the essentially vertical, uniform and stable alignment. Thus, blue or yellow color is reduced in a liquid crystal display of an STN mode to display a clear image of a high contrast by using an optical compensatory sheet, in which the discotic liquid crystal molecules are essentially vertically aligned (and preferably twisted).